Lithium-ion batteries are currently the most popular rechargeable batteries due to their high energy densities, relatively high cell voltages, and low weight-to-volume ratios. However, the voltage, charge capacity, battery life, and rechargeability of lithium-ion batteries have increased by relatively small increments over the past decade.
One issue in developing new battery technology is choosing suitable electrode composition(s). Electrochemically active metal oxides such as Fe2O3, Mn2O3 and Co2O3, graphite and silicon (Si), have long been investigated for use as anode materials for lithium-ion batteries because of their high theoretical capacities. Silicon, as well as many metal oxides typically exhibits a significant irreversible capacity loss in its first cycle and rapid capacity fade during cycling. A cycle refers to one charge and one discharge. Existing commercial anodes often have a specific capacity of between about 300 and 400 mAh/g when cycled at a charge/discharge rate of about 0.1C and often suffer from irreversible loss. Thus, it has been difficult to achieve a specific capacity of more than about 400 mAh/g when cycled at a charge/discharge rate of about 0.1C or higher over multiple charge/discharge cycles.
Furthermore, a large specific volume change commonly occurs during the cycling processes, which can lead to pulverization of the electrodes and rapid capacity decay. Furthermore swelling and contraction of silicon can affect the structure and properties of the electrodes.
It has been thought that the application of nanomaterials, particularly nanotubes, in batteries can offer vast improvements. Nanoparticles can include submicron (usually less than 1000 nm) carbon materials and/or nanoscale (usually less than 100 nm) carbon materials. The nanoparticles preferably have at least one dimension that is less than 500 nm, more preferably less than 100 nm and sometimes no greater than about 1 nm. Nanoparticles include, for example, nanospheres, nanorods, nanocups, nanowires, nanoclusters, nanofibers, nanolayers, nanotubes, nanocrystals, nanobeads, nanobelts and nanodisks.
Nanotubes are cylindrical structures formed by nanoparticles such as carbon-based nanoparticles. Nanotubes can be single-walled nanotubes (“SWNT”), multi-walled nanotubes (“MWNT”) which includes double-walled nanotubes (“DWNT”), or a combination of the same. When the nanotube is carbon-based the abbreviation can be modified by a “C-,” for example, C-SWNT and C-MWNT.
The structure of a single-walled carbon nanotube can be described as a single graphene sheet rolled into a seamless cylinder with ends that are either open, or capped by either half fullerenes or more complex structures such as pentagons. Multi-walled carbon nanotubes contain two or more nanotubes that are concentrically nested, like rings of a tree trunk, with a typical distance of about 0.34 nm between layers.
Nanomaterials have broad industrial applications, including transparent electrodes for displays and solar cells, electromagnetic interference shielding, and sensors. Nanoparticles, and specifically conductive nanoparticles of carbon, metals and the like, have been known and used for years in the fields of semiconductors and electronic devices. Examples of such particles and processes are provided in U.S. Pat. Nos. 7,078,276; 7,033,416; 6,878,184; 6,833,019; 6,585,796; 6,572,673; and 6,372,077. The advantages of having ordered nanoparticles in these applications are also well established (see, for example, U.S. Pat. No. 7,790,560).
Nanoparticles of various materials have been selected for a range of applications based on their various thermal and electrical conductivity properties. Among the nanoparticles often used are carbon nanoparticles: nanoparticles that are primarily composed of carbon atoms, including diamond, graphite, graphene, fullerenes, carbon nanotubes (including C-SWNT and C-MWNT), carbon nanotube fiber (carbon nanotube yarn), carbon fibers, and combinations thereof, which are not magnetically sensitive. Carbon nanoparticles include those particles with structural defects and variations, tube arrangements, chemical modification and functionalization, surface treatment, and encapsulation.
In particular, carbon nanotubes are very promising due to their chemical stability combined with electrical and thermal conductivity. Carbon nanotubes are long thin cylindrical macromolecules and thus have a high aspect ratio (ratio of the length over the diameter of a particle).
Nanoparticles, and in particular nanotubes, can enhance the strength, elasticity, toughness, electrical conductivity and thermal conductivity of various compositions. In certain applications the use of carbon nanotubes in materials is desirable yet hard to achieve. For example, nanotubes have a tendency to aggregate (also referred to as bundle or agglomerate), which impairs their dispersion. Non-uniform dispersion can give rise to a variety of problems, including reduced and inconsistent tensile strength, elasticity, toughness, electrical conductivity, and thermal conductivity. Generally, preparation of most materials incorporating single-walled carbon nanotubes and/or multi-walled carbon nanotubes has been directed at achieving well-dispersed nanotubes in polymers using methods such as mechanical mixing, melt-blending, solvent blending, in-situ polymerization, and combinations of the same. Attempts to create homogenous aqueous dispersions of single-walled and multi-walled carbon nanotubes have involved using certain water-soluble polymers that interact with the nanotubes to give the nanotubes solubility in aqueous systems such as the systems described in International (PCT) Publication No. WO 02/016257. However, these attempts have not been able to reach the desired dispersion due to multiple factors. Nanoparticles, particularly multi-walled, double-walled and single-walled carbon nanotubes, have a tendency to aggregate, which leads to non-uniform dispersion. Furthermore nanoparticles, and in particular nanotubes, often have relatively fragile structures that are damaged by many of the existing physical dispersion methods, such as mixing and intense or extended ultrasonication. In addition, it is believed that the geometrical shape of many nanoparticles and intramolecular forces contribute to a tendency for less uniform dispersion.
Previous attempts have been made to disperse nanoparticles and metal oxides in fluids (see, for example, U.S. Patent Application Publication No. US2008/0302998). However, these attempts did not address the proper dispersion of carbon nanomaterials and metal oxides and/or metal particles for desirable electrical conductivity and the formation of solid electrodes. Similarly, although U.S. Pat. No. 8,652,386 describes magnetic alignment of carbon nanotubes in nanofluids such as nanogreases and nanolubricants by employing metal oxides in the fluids, the prior art has been silent on the successful homogenous dispersion and integration of carbon nanomaterials with metal oxides and/or metal and/or metalloid particles in useful materials such as electrodes. Integration refers to when the ion absorbing particles are combined in an integrated fashion so that they are attached to the carbon nanoparticles.
U.S. Patent Application Publication No. US 2013/0224603 discusses electrodes comprising a mesa-porous graphene cathode and an anode comprising an active material for inserting and extracting lithium mixed with a conductive filler and/or resin binder. However, the methods disclosed have several limitations including construction of the anode in a conventional manner involving simple mixing of the components, and does not include any method of providing uniform dispersion of the active material or robust attachment of the active material to the conductive filler.
Similarly U.S. Pat. No. 8,580,432 discusses a composition for lithium-ion battery electrode applications comprising a lithium-ion conductive material in the form of submicron particles, rods, wires, fibers or tubes combined with nano-graphene platelets and incorporated in a protective matrix material. However, the patent does not disclose a method of ensuring uniform dispersion of the components or homogeneous distribution of the submicron additives and nano-graphene platelets in the matrix material.
Attempts to disperse carbon nanoparticles have included the use of nanotubes functionalized with magnetically sensitive groups including Ni-coated nanotubes. However, this approach failed as the functionalized nanotubes were found to suffer a decrease in electrical conductivity, strength and other mechanical properties in part due to the fact that once functionalized, the conjugated structure of the nanotubes is broken, which results in changes in surface properties.
Thus, it remains a serious technical challenge to effectively and efficiently disperse carbon nanotubes into a non-aggregating, preferably homogenous and uniform, integration with metal oxides and/or metal particles and/or silicon and/or silicon oxides, thereby providing materials having consistent electrical conductivity properties and/or improved capacitance for high performance energy storage systems.
There is a need for novel methods to develop essentially homogenous and uniform integration of electrically conducting carbon nanoparticles such as nanotubes and graphene for high performance electrodes in such a way that the integrity and functionality of the electrode is not affected by volume changes in the ion-absorbing component. This would potentially significantly enhance the capacity, performance, and lifetime of energy storage systems. In one of the embodiments described below, carbon nanoparticles are integrated with at least one of metals, metal oxides, silicon and/or silicon oxides for use as electrodes.